Satellite images suggest Mars had vast Earth-like ocean

Scientists believe the ocean stretched around the planet's northern hemisphere and held 10 times less water than all the Earth's oceans combined.

It may also have provided a cradle for the emergence of extraterrestrial life.

Scientists have long debated whether oceans, seas, lakes or rivers existing on Mars long ago.

Satellite images reveal features on the planet resembling river valleys and flood plains created by flowing water. However, there are alternative explanations for their formation, such as volcanic activity.

The new research, published in the journal Nature Geoscience, provides the best evidence yet that an enormous ocean once existed on Mars.

US scientists from the University of Colorado at Boulder, based their findings on a study of what appear to be ancient river delta deposits and valley networks.

They identified 52 delta regions fed by numerous river-like systems. All lay at about the same height, suggesting that they marked the shoreline edge of a huge river-fed ocean.

The researchers, led by Dr Gaetano Di Achille and Professor Brian Hynek, believe the ocean covered about 36 per cent of the planet.

They wrote: "We suggest that the level reconstructed from the analysis of the deltaic deposits may represent the contact of a vast ocean covering the northern hemisphere of Mars around 3.5 billion years ago.

"Our findings lend credence to the hypothesis that an ocean formed on early Mars as part of a global and active hydrosphere."

Another study led by Prof Hynek and reported in the Journal of Geophysical Research – Planets identified roughly 40,000 river valleys on Mars.

A key unanswered question is: where did all the water on Mars go? Some scientists believe much of it may remain in frozen deposits underground.

River delta sediments on Earth rapidly cover up organic carbon and other biomarkers of life. For this reason, Martian delta systems are likely to be key targets for future exploration.